An esterification reaction that is one of representative reactions of organic synthesis is an important reaction that has high utility value in views of environmentally friendly chemical processes, and many studies thereof have been reported.
For example, in order to synthesize biodiesel having high quality from vegetable-oil including an oleic acid and a stearic acid, an esterification reaction is applied using a catalyst, an importance of which has been embossed (Nature 438, 178, 2005).
In general, the esterification reaction frequently uses a coupling agent and an auxiliary activator in an amount of 1 equivalent or more on the basis of reactants, and after the reaction, since a great amount of side products are generated, an additional purification process such as distillation or recrystallization is required.
In addition, in a real reaction, if any one of a carboxylic acid and alcohol is not used in an excessive amount, there is a problem in that it is impossible to obtain efficiently ester (for example, see Synthesis, 1978 p. 929, Chem. Lett, 1977 p. 55, Chem. Lett. 1981 p. 663, Tetrahedron. Lett. 28, 1987 p. 3713, J. Org. Chem. 56, 1991 p. 5307). However, as described above, it is ideal to directly perform esterification from the carboxylic acid and alcohol in the same or similar mole number while the reactant is used in an excessive amount.
Accordingly, currently, by using the carboxylic acid and alcohol in almost the same mole number, a catalyst for synthesizing ester has been developed.
For example, Korean Patent Application Laid-Open No. 2003-0042011 relates to a method for producing an ester condensate, and a synthesis reaction of a monomer ester or thioester, or polyester or polythioester by using a tetravalent hafnium compound represented by hafnium chloride (IV), particularly, hafnium chloride (IV) (THF)2 or hafnium (IV)t-butoxide as a (poly)condensation catalyst.
In addition, Korean Patent Application No. 2006-0065726 (Registration No.: 0729714) discloses an example of esterification reaction by using a catalyst that includes zirconium(IV) compound and/or hafnium(IV) compound and iron compound and/or gallium compound. At this time, as the zirconium(IV) compound, a compound that is represented by Zr(OH)a(OR1)b (R1 is an acyl group or an alkyl group, a and b are each an integer in the range of 0 or 1˜4, and a+b=4), or zirconium(IV) halogenate is used.
However, even though the hafnium compound, the zirconium halogenate, and the zirconium compound have excellent performance, there is a disadvantage in that it is not well dissolved in a nonpolar solvent such as heptane, octane, and toluene at normal temperature because of a characteristic of inorganic salt compound. In addition, after the reaction is finished, while the post-treatment is performed, catalyst residuals remain on a wall of a reactor. Thus, there is a problem in that it is difficult to remove the residuals. In the case of when synthesis is performed by using a great amount of reactant in a factory, there is a problem in that a process for washing a vessel is further required, and after the reaction is performed, many problems, for example, an additional removal of catalyst residuals that are not dissolved in a final solution, are included.
In addition, Korean Patent Application Laid-Open No. 2005-0050549 discloses an example of production of carboxylic acid ester by reacting a carboxylic acid and a monohydroxy compound in the presence of a catalyst after the zirconium catalyst is produced by reacting the monohydroxy compound and a Zr(OR)4 type of (R is an alkyl group or an aryl group) zirconium compound with each other. At this time, a ligand that is bonded to a zirconium element is a form where an oxygen atom is included in one single molecule (monodentate).